[@ejoopasco Participant 2 months, 1 week ago]

So this is closed now.

[@ejoopasco Participant 2 months, 1 week ago]

I stumbled upon my old threat and I want to clarify in case it helps someone:

The problem was in our Network. As soon as 8 people logged in from inside our institutes wlan, the problems occured.

People from outside our network could still easily log in. So no Buddypress problem at all.

[@ejoopasco Participant 1 year, 8 months ago]

Hope to help someone else stumbling upon this. problem I identified the cause of this behaviour.
In Settings -> BuddyPress -> Options I activated

Allow visitors to request site membership. If enabled, an administrator must approve each new site membership request.

If I deactivate that, the mail gets send.
I guess that is intended. Maybe I need to make a feature request now.